Falling through air

Class practical

Ideas about terminal velocity and about streamlining can be quite difficult. This gives students some practical experience on which to build their understanding.

Apparatus and materials

Table-tennis ball

Sheets of paper (A4 is suitable)

Parachute, improvised with paper or cloth

Health & Safety and Technical notes

Students must NOT be allowed to stand on stools or benches in this activity. Serious accidents have happened as a result. Parachutes can often be dropped down stairwells providing pupils are adequately supervised.

Procedure

a Hold a sheet of paper horizontally and let it fall to the ground. Watch how it moves.

b Fold the paper to make a tray, with sides about 2 cm high. Drop the tray and watch it fall. Try it both ways up.

c Discuss the reasons why the paper falls in different ways.

d Drop a small ball.

e Make a parachute from cloth or paper, with threads hanging from its edges.

f Tape the ball to the parachute. Drop the ball and its parachute. Discuss how the parachute makes a difference to how the ball falls.

Technical notes

1 The plain sheet of paper flutters down when allowed to fall but the tray falls more steadily. As a tray, the 'right' way up, it not only presents a smaller surface area to the air but also encourages a more even air flow past it. It is more 'streamlined'.

2 Forces of resistance in fluids, sometimes called forces of fluid friction, increase with speed. They bring a falling body to a constant speed, the terminal velocity.

When terminal velocity is achieved then the upwards forces (due to the resistance to motion and due to any flotation effect of the fluid) are equal to the weight of the body. (This is what happens with parachutes and raindrops. More massive bodies need to fall longer distances before they reach terminal velocity.)

There are in fact at least two different kinds of fluid resistance: the kind associated with streamline motion that depends on the speed of the body, and the kind that leaves a wake of vortex motion and involves resistance that varies with the square of the speed.

3 Students can devise their own additional investigations, which could include:

dropping a tray from different heights

working with trays carrying different loads, or with holes in the tray, or trays of different cross-sectional area (or even with the paper screwed up as a ball)

dropping different numbers and hence different weights of paper cups, or different sizes of paper cup (as used for baking cupcakes)

varying the area of the parachute, the number of holes in it, the number of strings and the shape.